Conventional systems for controlling the mass flow rate of fine materials, such as particulate coal, supplied as a fuel into, for example, a furnace such as a coal-fired boiler, generally employ load-cell-based, weight-rate measuring systems to control the gross mass flow rate of the coal to boilers. Such a system is described in the Shimada et al. U.S. Pat. No. 4,490,077. In U.S. Pat. No. 4,838,738 issued to Assignee herein on Jun. 13, 1989, the fast response of a mass flow rate-indicating device is combined with the longer term accuracies in changes in the total mass from weight cells to yield rapid and more accurate loss-in-weight measurements than conventional systems.
U.S. Pat. No. 3,635,082 to Prellwitz et al. describes the determination of mass flow from velocity and density measurements of a coal and gas stream using two capacitance transducers mounted a known distance apart in the supply line between a coal storage vessel and furnace. The measurement system described by this patent relies on large signal variations, i.e., large changes in flow density, including inducing a change in flow density by injecting a burst of compressed gas into the supply line to produce a marker gap to observe "slugs" of coal in a gas stream in an industrial process. Such a system would not be compatible with Assignee's coal gasification process which requires a uniform mass flow rate of coal introduced to the gasifier over periods of time of approximately 5 seconds.
Other conventional systems control the mass flow rate by determining the mass flow rate indirectly via optical measurement of the coal concentration within a conduit leading to the furnace. Accurate control of mass flow by optical measurements, such as radiation absorption of infrared, UV or visible light, is limited to applications of low coal density suspensions, say less than 10 kg/cubic meter, since light must be transmitted through the coal mixture present in the conduit. Knowledge of the particle size distribution is also required. U.S. Pat. No. 4,049,394 to Gernhardt et al. describes a system for maintaining a predetermined volumetric ratio between fine-particle fuel and a gasification agent which are fed separately into a reactor. This system utilizes the principle of absorption of electromagnetic radiation by the fuel. First, the coal entrains gas as it passes from a storage vessel into a transport line. Additionally, the carrier gas is introduced to assist the coal in discharging from the vessel to the transport line en route to the gasifier. Since it is the total gas stream in the transport line in addition to other factors which govern which govern the mass flow rate, the invention described by this patent could not be used to control the mass flow rate of coal to a gasifier within the desired accuracy, say plus or minus 2 percent, operated with varying suspension densities of 100-800 kg/cubic meter having a variable coal particle size distribution, which are characteristic of the various coal types and various flow conditions required by the instant invention. Typical radiometric density measurements, although accurate, are too slow to ensure constant mass flow rates over short periods of time. Forster et al., U.S. Pat. No. 4,270,558, discloses a radiometric density measurement which compensates for the carrier gas. In Assignee's co-pending application Ser. No. 098,179 filed Sep. 18, 1987, the suspension density and mass flow rate of a particulate solids and gas mixture transported to a reactor is controlled by using a radiation source and detector. The suspension density is measured and compared to a preselected value. The result is converted to a control signal which may be used to control venting from the vessel, and/or the amount of aeration gas supplied to the lower portion of the vessel in order to maintain the suspension density at the preselected value and provide a constant mass flow rate. However, such systems do not disclose rapid manual or automatic on-line recalibration of mass flow rate controlling systems within a few seconds to accommodate changing operating conditions as required by the present invention. The present invention is directed to overcoming these problems in the prior art.
Applicant is not aware of any prior art which, in his judgment as one skilled in this particular art, would anticipates or render obvious the present invention. However, for the purpose of fully developing the background of this invention, and establishing the state of requisite art, the following art is set forth: U.S. Pat. Nos. 3,635,082; 4,490,077; 4,049,394; 4,838,738 and 4,270,558.